It outputs "lib: lib". No arch triplet. Is this expected?
Yes; you probably want ${CMAKE_INSTALL_FULL_LIBDIR}.
http://www.cmake.org/cmake/help/v3.1/module/GNUInstallDirs.html
https://anonscm.debian.org/cgit/pkg-opencl/beignet.git/tree/debian/patches/Enable-multiarch.patch?id=a4be256b30625db8829a5d752dc71aa4eb61229b